1. Field of the Invention
This invention relates to the fields of electrically alterable circuit elements, semiconductors, and electrical memory devices. More particularly this invention relates to a technique for electrically changing the properties of a lateral polycrystalline silicon (polysilicon) zener diode. This process renders the device particularly useful as an analog or digital electrical memory element.
2. Background of the Invention
Polysilicon diodes and resistors as well as nichrome links and other devices have been used as fusable links for PROMs (Programmable Read Only Memory) in the recent past by injecting large currents into the devices to create open or short circuits (primarily by altering the contact metalization of the device). This type of programming often results in structural damage to the devices or substrate because of the physical stresses induced by the programming process. Also, contamination of the integrated circuit can take place as a result of structural damage to the metalization and surrounding structures and ruptures may occur due to venting of gasses as a result of the high temperatures created in the programming process. These factors ultimately degrade the reliability of the PROM resulting in premature failure of the electronic device utilizing the PROM.
It is also possible to convert vertical zener diode structures to resistors for use as a memory element by injecting large currents thereby causing the contact metalization to diffuse through the diode structure. This is commonly known as "zener zapping" and causes many of the same reliability problems mentioned above.
In addition to these problems, none of the known link blowing processes are reversible and many of the known structures require highly specialized integrated circuit processing steps thereby radically increasing the cost. Furthermore, none of the known structures and processes lend themselves to use as a non-volatile analog memory element due to the completely binary nature of the programability. Analog memory elements may by their very nature store many bits of information as for example a single voltage level and greatly increases the information density possible in a single memory device. Binary memory elements are only capable of storing a single binary digit (bit) per memory element.